Illustration of the four different types of glial cells found in the central nervous system: ependymal cells, astrocytes, microglial cells, and oligodendrocytes

Devic's disease was associated in the past with many systemic diseases, based on anecdotal evidence. The discovery of NMO-IgG (anti-AQP4) has opened a new way into the research for the causes. Currently, three principal causes are accepted:

A first subset of patients is anti-AQP4+. In them, the cause of the neuromyelitis optica (understood as the syndrome) is an autoimmune aquaporin-4 channelopathy, due to these specific autoantibodies.[5] In these cases, astrocytes are the victims of the autoimmune attack.

In a third subset of patients, NMO is due to the presence of anti-neurofascin (anti-NF) auto-antibodies which damage the Ranvier nodes of the neurones.[7] These antibodies are more related to the peripheral nervous demyelination, but they were also found in NMO.[8]

Rarely, it has been reported that some courses of anti-NMDAR are consistent with NMO.[9] It is not excluded the possibility that new kinds of autoantibodies could appear in NMO.

Aquaporin 4 is found in the astrocytes that surround the blood–brain barrier (BBB), a system responsible for preventing substances in the blood from crossing into the brain. The blood–brain barrier is weakened in Devic's disease, but it is currently unknown how the auto-antibodies cross the BBB. Some reports point to the metalloproteinase-2 and interleukin-6 as culprits for the BBB failure.[11] There is broad consensus that AQP4/NMO-IgG initially enters the brain via BBB-deficient sites such as area postrema, where access to CSF occurs.[12]

There are "normal appearing areas" in NMO but compared to patients with MS, NAWM damage is less extensive.[13]

Most research into the pathology of Devic's disease has focused on the spinal cord. The damage can range from inflammatory demyelination to necrotic damage of the white and grey matters. The inflammatory lesions in Devic's disease have been classified as type II lesions (complement-mediated demyelinization), but they differ from MS pattern II lesions in their prominent perivascular distribution. Therefore, the pattern of inflammation is often quite distinct from that seen in MS.[2][14]

About the presence of anti-AQP4 autoantibodies, some researchers have pointed out that some other cases could be paraneoplastic.[15] It seems also clear that lupus can produce NMO-IgG autoantibodies sometimes, leading to some cases of lupus-derived NMO.[16]

In Anti-AQP4 positive variants, CNS astrocytes, which are the basis for the glymphatic system[19] are the target of the autoimmune attack. NMO-IgG-negative cases are less understood. It seems currently that astrocytes are spared in these IgG negative cases.[20]

Currently, the second most documented auto-antibody in NMO is anti-MOG autoantibody.

MOG antibodies are currently considered mostly absent in similar diseases like multiple sclerosis.[21] Therefore, it can be said that anti-MOG is a group contained inside AQP-Negative NMO.[22]

Together with the anti-AQP4 they form the wider part of the NMO spectrum. They classify the NMO cases in four classes, according to the presence or absence of any of these two main auto-antibodies.[23]

The clinical course and the response to therapy is different for these groups, showing a better prognosis for those in the NMO-Ab(−)/MOG-Ab(−) group, and a worse prognosis for those in the NMO-Ab(+)/MOG-Ab(+) group.[23] The MOG-related neuromyelitis optica can be radiologically identified by the conus involvement. Myelin-oligodendrocyte glycoprotein antibody–positive patients were more likely to have conus involvement on spinal magnetic resonance imaging.[24]

The Mayo Clinic proposed a revised set of criteria for diagnosis of Devic's disease in 2006. Those new guidelines require two absolute criteria plus at least two of three supportive criteria.[26] In 2015 a new review was published by an international panel[3] refining the previous clinical case definition but leaving the main criteria unmodified:

Since the discovery of AQP4 autoantibody, it has been found that it appears also in patients with NMO-like symptoms that do not fulfill the clinical requirements to be diagnosed NMO (Recurrent and simultaneous optic nerve and spinal cord inflammation).[27]

The term NMOSD (NMO Spectrum Disorders) has been designed to allow incorporation of cases associated a non-AQP4 biomarkers.[3] Therefore, it includes all the clinical variants due to anti-AQP4 plus other non-related but clinically similar syndromes like antiMOG associated encephalomyelitis. Some cases with MOG+ and AQP4+ antibodies have been found.[3]

The collection of these condition has been named "neuromyelitis optica spectrum disorders" (NMSD) and they are expected to respond to the same treatments as standard NMO.[28] Some authors propose to use the name "autoimmune aquaporin-4 channelopathy" for these diseases,[5] while others prefer a more generic term "AQP4-astrocytopathy" that includes also problems in AQP4 with a non-autoimmune origin.[29]

Whether Devic's disease is a distinct disease or part of the wide spectrum of multiple sclerosis is debated.[34] Devic's disease differs in that it usually has more severe sequelae after an acute episode than standard MS, MS infrequently presents as transverse myelitis, and oligoclonal bands in the CSF, as well as white matter lesions on brain MRI, are uncommon in Devic's disease, but occur in over 90% of MS patients.[35]

Recently, AQP4 has been found to distinguish standard multiple sclerosis from neuromyelitis optica, but as MS is a heterogeneous condition,[36] and some MS cases are reported to be Kir4.1 channelopathies[37] (autoimmunity against the potassium channels) it is still possible to consider NMO as part of the MS spectrum. Besides, some NMO-AQP(−) variants are not astrocytopathic, but demyelinating.[38]

AQP4-Ab-negative NMO presents problems for diagnosis. The behavior of the oligoclonal bands respect MS[clarification needed] can help to establish a more accurate diagnosis. Oligoclonal bands in NMO are rare and they tend to disappear after the attacks, while in MS they are nearly always present and persistent.[41]

It is important to notice for differential diagnosis that, though uncommon, it is possible to have longitudinal lesions in MS.[42]

Other problem for diagnosis is that AQP4ab in MOGab levels can be too low to be detected. Some additional biomarkers have been proposed.[43][44]

Plasmapheresis can be an effective treatment[31] when attacks progress or do not respond to corticosteroid treatment. Clinical trials for these treatments contain very small numbers, and most are uncontrolled, though some report high success percentage.[45]

Hematopoietic stem cell transplantation (HSCT) is sometimes used in severe cases of NMO. Currently available data suggest that this procedure can reduce inflammatory activity in the short term, but a clear majority of the patients will relapse within 5 years.[54]

Normally, some measure of improvement appears in a few weeks, but residual signs and disability may persist, sometimes severely.

The disease can be monophasic, i.e. a single episode with permanent remission. However, at least 85% of patients have a relapsing form of the disease with repeated attacks of transverse myelitis and/or optic neuritis. In patients with the monophasic form, the transverse myelitis and optic neuritis occur simultaneously or within days of each other. On the other hand, patients with the relapsing form are more likely to have weeks or months between the initial attacks, and to have better motor recovery after the initial transverse myelitis event. Relapses usually occur early, with about 55% of patients having a relapse in the first year and 90% in the first five years.[2]

It is possible that the relapsing form is related to the antiAQP4+ seropositive status and the monophasic form related to its absence[55] Unlike multiple sclerosis, Devic's disease rarely has a secondary progressive phase in which patients have increasing neurologic decline between attacks without remission. Instead, disabilities arise from the acute attacks.[2]

Approximately 20% of patients with monophasic Devic's disease have permanent visual loss, and 30% have permanent paralysis in one or both legs. Among patients with relapsing Devic's disease, 50% have paralysis or blindness within five years. In some patients (33% in one study), transverse myelitis in the cervical spinal cord resulted in respiratory failure and subsequent death. However, the spectrum of Devic's disease has widened due to improved diagnostic criteria, and the options for treatment have improved; as a result, researchers believe these estimates will be lowered.[2]

The prevalence and incidence of Devic's disease has not been established, partly because the disease is underrecognized and often confused with MS.[2] Devic's disease is more common in women than men, with women comprising over two-thirds of patients and more than 80% of those with the relapsing form of the disease.[2]

A retrospective study found that prevalence of NMOsd was 1.5% inside a random sample of neurological patients, with a MS:NMOsd ratio of 42.7. Among 13 NMOsd patients, 77% had long spinal cord lesions, 38% had severe optic neuritis and 23% had brain or brainstem lesions. Only 56% had clinically definite NMO at follow-up.[56]

According to the Walton Centre in England, "NMO seems to be present across the world unlike MS, which has a higher incidence in temperate climates and white races. Africans and Asians especially in Far East may have a higher risk of NMO, although the exact incidence of this disease is unknown, making specific conclusions difficult". Although many people who have Devic's disease were initially misdiagnosed with MS, 35% of African Americans are often misdiagnosed with MS when they really have NMO.[citation needed]

Devic's disease is more common in Asians than Caucasians. In fact, Asian optic-spinal MS (which constitutes 30% of the cases of MS in Japan) has been suggested to be identical to Devic's disease (differences between optic-spinal and classic MS in Japanese patients). In the indigenous populations of tropical and subtropical regions, MS is rare, but when it appears, it often takes the form of optic-spinal MS.[57]

The majority of Devic's disease patients have no affected relatives, and it is generally regarded as a nonfamilial condition.[2]

In 2002, Mayo Clinic researchers identified an humoral mechanism, targeting a perivascular protein, as the culprit for NMO [14] and in 2004 an unknown specific autoantibody was found.[63] In 2005 they identified the aquaporin 4 protein as the target of the disease, and developed a test to aid in the diagnosis of Devic's disease by detection of an antibody, AQP4-IgG, in the blood.[19]

Later some other auto-antibodies have been found in NMO AQP4-negative cases, like anti-MOG IgG, but some NMO Anti-AQP4-negative cases still remain idiopathic.

Since the discovery of AQP-4 involvement, some research studies have focused on targeted treatment aimed at anti-aquaporin 4 antibodies. The most established method for antibody removal is plasmapheresis. A number of drugs are being studied: aquaporumab (non-pathogenic antibody blocker of AQP4-IgG binding), sivelestat (neutrophil elastase inhibitor), and eculizumab (complement inhibitor).[64]

There is little research into the primary causes of the Anti-AQP4 auto-antibodies. It has been noticed that some cases could be paraneoplastic.[15]

In addition, several NMO variants have been discovered with antibodies other than those for AQP4, turning NMO into a heterogeneous disease. Six different patterns of damage have been reported in NMO, raising the possibility of six different types of auto-antibodies. As of 2019 only three of them are known.[65]

Other auto-antibody under research is flotillin. It has been found in seronegative NMO and some MS patients[67]

Finally, other proteins under study are Connexin 43 and anti-AQP1[68]though, as of 2015, there are only initial reports about the involvement of these proteins[27][29]

The group AQP+/MOG+ is very small and it can be considered a coincidence of two independent problems in the same person. Assuming these cases could be verified, currently five different kinds of NMO are being considered:

NMO derived from an autoimmune channelopathy (AQP4-Ab+), around 80% of the cases

Some cases of NMO are not due to auto-antibodies. They constitute an overlap between NMO and Multiple sclerosis. Some statistical studies show that antibody negative NMO can be classified in three groups, and that this classification has a pathogenic meaning.[70]